1. Field of the Invention
The invention relates to an antenna array that is part of a radio frequency Identification (RFID) system for wireless or contact-free data transmission, particularly for reading from and writing to wireless contact-free data carriers, such as transponders.
2. The Prior Art
According to the state of the art, an RFID system comprises an RFID antenna that consists of at least one antenna loop that represents an inductance and is formed from one or more windings, an adaptation circuit; a read/write station having an integrated transmission, a receiver, and control unit; and a connecting line between the read/write station and the RFID antenna.
The RFID antenna of an RFID system has the following tasks: On the one hand, the transmission of power to the transponder, and on the other hand, the transmission of data to and from the transponder. The power and data transmission is based on the magnetic coupling of the alternating fields of the reader and the transponder in the close vicinity of the antenna.
A requirement for an RFID reader antenna is the power transmission to the transponder. For this purpose, the reader antenna, in turn, is supplied with power via a transmitter. To optimally transmit the power from the end stage of the reader to the antenna, the reader end stage and the reader antenna must possess the same input and output resistance, respectively. An RFID reader antenna therefore requires a certain input impedance, so that the power is optimally transmitted to the antenna from the reader end stage.
Furthermore, the reader antenna must be tuned as well as possible to the operating frequency of the RFID system, to achieve a high current and therefore a high magnetic field intensity.
If the resonance frequency of the reader antenna is tuned to the operating frequency of the RFID system, there is a high power transmission at a high quality for the reader antenna.
An adaptation circuit can adapt the input and output resistance, respectively, and balance the reader antenna with the operating frequency. This circuit is generally located in the direct vicinity of the antenna loop.
According to the state of the art, transponders consist of an integrated microelectronic component (IC) a resonance capacitor and an antenna coil, whereby the resonance capacitor is frequently already integrated into the microelectronic component. The antenna coil and the resonance capacitor form an electrical oscillating circuit and are, for example, tuned to the operating frequency of 13.56 megahertz (MHz).
If a transponder gets into the detection region of the reader antenna, the transponder receives power for operating the integrated circuit (IC), via the magnetic coupling with the antenna. The amount of the power is dependent on the field intensity, for example, on the number of field lines that penetrate the transponder, and the angle of the field lines to the transponder. The transponder receives the maximal power in the case of an angle of 90 degrees between field lines and transponder. If the angle between field lines and transponder is very acute or actually 0 degrees, the transponder is not penetrated by any field lines and therefore also does not receive any power.
If the power is sufficient, the microelectronic component is set into its base state and begins to work.
The range in which the transponder has sufficient power and can communicate with the RFID system is called the detection region.
The size of the detection region is determined by the following factors: antenna area, antenna shape, and current in the antenna loop (antenna current). The current, in turn, is dependent on the quality of the oscillation circuit, the output power of the RFID reader, and the inductance of the antenna loop, and reaches its maximum when the antenna is tuned to the resonance frequency.
To achieve the largest possible detection region, the largest possible antenna area would consequently have to be operated at the greatest possible antenna current. In practice, however, this is limited by various restrictions.
On the one hand, the output power of the end-stage amplifier of the RFID reader is limited, to keep the effort and expense as low as possible. Because of the antenna or of the adaptation circuit that can be heated during use, as well as experience the high voltages at the components in the resonance circuit, these effects also make the design and the development of cost-advantageous solutions for high transmission power more difficult.
The dependence of the magnetic field intensity on the current and the size of the antenna can be represented, fundamentally, using the following formula for a round antenna:
      H    ⁡          (      x      )        =            I      ·      N      ·              r        2                    2      ⁢                        (                                    r              2                        +                          x              2                                )                          3          /          2                    Wherein:    H: magnetic field intensity    x: distance between the plane of the antenna and the transponder    I: current through the antenna    N: number of windings of the antenna    r: radius of the round antenna
If the distance x between transponder and antenna loop is chosen to be equal to zero (transponder lies in the same plane as the antenna loop), the formula can be simplified as follows:
      H    ⁡          (      x      )        =            I      ·      N              2      ⁢      r      
The formula shows that the field intensity in the antenna center decreases at 1/r. If one assumes that the RFID reader is able to drive only a certain maximal current into an antenna array, it becomes clear that starting from a certain size of the antenna, the magnetic field intensity in the center of the antenna is no longer sufficient to operate a transponder.
On the other hand, the inductance becomes greater and greater for large antennas, as a function of the conductor length, for example, the area surrounded by the conductor. With higher frequencies (for example at 13.56 MHz), there are very small capacitances. Thus, according to the following equation, with these small capacitances it would be necessary to balance the antenna with the required resonance frequency.
The calculation of the resonance frequency can occur using the following formula:
      f    res    =      1          2      ⁢      π      ⁢                        √          L                ·        C                f: frequency    L: inductance    C: capacitance.
These small capacitances in the resonance circuit make the antennas difficult to balance, and the sensitivity to changes in the ambient conditions becomes greater.
Large antennas and high transmission power furthermore lead to a disadvantage of exceeding the valid limit values of the national radio interference regulations.
Also, it is not possible to determine the position of the transponder within the large antenna area.
Another problem of a large antenna area occurs if the RFID reader is supposed to be able to communicate with a transponder even if a large number of transponders are located in the detection region of the reader antenna at the same time. In this case, the RFID power is divided up over the large antenna area, causing the local field intensity at each point in the detection region to drop. In this case, the resonance frequency of the transponders can change because of the reciprocal coupling with one another. This can occur particularly if the transponders are spaced apart at short distances from one another, thereby causing the field intensities that the transponder requires for operation to increase.
Furthermore, simple large antennas having a large area have the disadvantage that the transponders can only be read in one orientation, since the field lines exit from the antenna area perpendicular to it, and must also penetrate the transponder as perpendicular as possible, so that the necessary power is transmitted to the transponder.
A solution for the disadvantages discussed is described in a prior art reference WO 03/026067 A1, which was published as U.S. Pat. No. 2003/0052783 to Sizman on Mar. 20, 2003, the disclosure of which is hereby incorporated herein by reference. The desired detection region of an antenna is built up by way of the sum of the detection regions of several small antenna loops. Here, the individual antenna loops are brought together, in pairs, with an adaptation circuit, and additional antennas are connected with the RFID reader via power splitters or power dividers.
Disadvantages of the solution described are that the balancing of the antennas to the resonance frequency is very difficult, since the antennas, which are at small distances from one another, can reciprocally influence one another as a function of their distance from one another. Furthermore, gaps in the detection field occur between the antennas, or the range of the detection region is significantly lower at these locations. Thus, a continuous detection region exists only in the vicinity of an antenna, since the detection regions of the adjacent antennas only overlap slightly.
Another solution is proposed in another set of prior art references (DE 201 10 926 U1 and DE 299 21 752 U1). There, an active antenna connected with the reader is supplemented with one or more passive antennas. Here, the adaptation circuit of a passive antenna generally consists mainly of a capacitor that tunes the antenna to the resonance frequency. The antenna has no electrical connection with the reader and receives its power, just like the transponders do, only by way of the magnetic coupling with the active antenna.
With this solution, again, gaps occur in the detection region of the structure, due to superimposition of the magnetic field lines. This is because the field lines have different directions and phase positions relative to one another. As a result, the transponder has few field lines flowing through it if it is oriented parallel to the antenna, and therefore does not receive sufficient power.
Furthermore, the signal strength of the response telegrams from the transponders is reduced by the coupling factor between the active and the passive antenna, if a transponder is located outside of the detection region of the active antenna but in the vicinity of the passive antenna. This can have the result that although the transponder has sufficient power, it cannot be read.
If the antennas are overlapped, as described in a prior art reference (EP 1 298 573 A2), electromagnetic coupling occurs between the individual antenna loops, thereby changing the resonance frequency of the antennas and causing the transmission output of the reader to be distributed over all the coupled antennas, thereby resulting in a general reduction or a detection region having gaps.
Antennas with a small distance from one another or overlapping antennas have the property of being strongly coupled into one another. This has the result that balancing of the antennas is difficult and complicated, since they reciprocally influence one another.
When the antenna is put into operation, part of the transmission power goes into the adjacent antennas, because of the coupling. This power is lost to the transmission antenna, and this results in reduced field intensity values and reading ranges.
If the phase position of the currents of two antennas is the same-phase in the overlap region, holes occur in the detection region, in the region of the overlap, since the field lines are counter-current there.
Nevertheless, the division of a large antenna area into smaller individual antennas is already described in a prior art reference (WO 03/026067 A1 and EP 1 298 573 A2), whereby fundamental advantages of overlapping antennas are also already mentioned in EP 1 298 573 A2. Other Patents are known, for example, EP 01 86 483; U.S. Pat. No. 6,703,935 to Chung (the disclosure of which is hereby incorporated herein by reference); WO 03/090310 to Yang; and EP 06 54 840.
The technical problem underlying the invention is to create an antenna array for large, cohesive detection spaces, which avoid the disadvantages mentioned above.